My beginner game - would love to hear criticism.

I got into game programming about two months ago. I wanted to share with you the second game I created (the first was a Pong variation with missiles and shit).

It's super basic, and I did all the graphics in MSPaint ;) Still I'd like to hear your criticism. Any problems with the game, or ideas how to improve it game-design-wise or programming-wise would be great.

I made this game in Java.

I'm probably not going to put more effort into this game. I want to hear your criticism for future games.

Also, rarely the game has some technical issues on some computers. If the game doesn't run as it's supposed to on your computer, please tell me.